Magic (cryptography) - Other Japanese Ciphers

Other Japanese Ciphers

PURPLE was an enticing, but quite tactically limited, window into Japanese planning and policy because of the peculiar nature of Japanese policy making prior to the War (see above). Early on, a better tactical window was the Japanese Fleet Code (an encoded cypher), called JN-25 by U.S. Navy cryptanalysts. Breaking into the version in use in the months after December 7, 1941 provided enough information to lead to U.S. naval victories in the battles of the Coral Sea and Midway, stopping the initial Japanese advances to the south and eliminating the bulk of Japanese naval air power. Later, broken JN-25 traffic also provided the schedule and routing of the plane Admiral Isoroku Yamamoto would be flying in during an inspection tour in the southwest Pacific, giving U.S Army Air Corps pilots a chance to ambush the officer who had conceived the Pearl Harbor attack. And still later, access to Japanese Army messages from decrypts of Army communications traffic assisted in planning the island hopping campaign to the Philippines and beyond.

Another source of information was the Japanese Military Attaché code (known as JMA to the Allies) introduced in 1941. This was a fractionating transposition system based on two-letter code groups which stood for common words and phrases. The groups were written in a square grid according to an irregular pattern and read off vertically, similar to a disrupted columnar transposition. Then the letters were superenciphered using a pre-arranged table of alphabets. This system was broken by John Tiltman at Bletchley Park in 1942.